No, it's not your imagination. It's getting hotter every year - and scientists predict the urban heat island effect, in which more buildings and pavement cause temperatures to rise, will only get worse.
From 1969 to this year, as Pima County grew from about 300,000 to nearly a million people, the average nighttime low temperature at a north-side Tucson monitoring station rose 3.5 degrees, University of Arizona researchers say.
Over the next 40 years, the trend is likely to worsen or at least stay about the same, depending on how fast the regional "Sun Corridor" grows and lays down pavement between Prescott and Southern Arizona, a new Arizona State University study projects.
The average nighttime low could rise another 2 to 7 degrees in that period, the study found, using computer-modeled scenarios of possible growth patterns.
And the more desert that gives way to concrete and asphalt, the less rain we'll see, the ASU study found. More buildings and pavement means less evapotranspiration of moisture from trees, shrubs and other plants into the atmosphere, where it converts to precipitation.
The result? By mid-century, look for up to a 12 percent drop in rainfall, primarily in summer monsoons, in a region already predicted to have drier winters due to greenhouse gas emissions.
heat endures at night
Benny Ochoa, a laborer and construction worker, grew up in the Old Fort Lowell area in the 1950s when its roads were dirt and few houses existed.
But this inconvenience had a bright spot: His family could make it through a hot summer night simply by leaving the doors and windows open.
Now 73, he lives near South Alvernon Way, between East Broadway and East 22nd Street. In the summer, he can still feel the heat on the pavement in front of his house after the sun goes down.
"When the temperature is real high during the day, it seems to me it takes a lot longer to cool off now," he said. "I sit outside, but there are just nights where it's just too stuffy, and I go inside where it is cool."
The lead author of the ASU study, Matei Georgescu, likens the land surface to a sponge.
When it rains, the soil soaks up and stores the water. The next day, that moisture evaporates into the sky, making the next storm potentially more intense.
Put a blanket of pavement atop this sponge, and it can no longer store the water, he said - "There's no more evaporation the next day."
Overall, the ASU study concludes that heat island impacts on Southwestern urban temperatures will likely be more severe than those of global warming, which most climate scientists believe is caused by emissions of carbon dioxide and other greenhouse gases from fossil fuel burning.
By 2050, average temperatures in this area are expected to rise another 1 to 4 degrees due to greenhouse gases, says a recent Southwest Climate Assessment report prepared by more than 100 researchers around the West. If both kinds of warming occur at once, temperatures could rise an uncomfortable to stifling 3 to 11 degrees total.
"Most of our policies today are focused on limiting greenhouse gases," said Georgescu, an ASU sustainability scientist and the new study's lead researcher. "We need to consider not just greenhouse gas emissions policy but policies related to sustainable development" that could help the heat island effect.
"There's been plenty of urban heat island research. By itself, this paper was not novel. What made it interesting is that the heat island effect can be as or more important than climate change," he said.
Exactly what sustainable development means is a matter of continuing debate. But Melanie Lenart, of the University of Arizona's Department of Soil, Water and Environmental Science, said the new study clearly drives home the importance of both natural landscaping such as urban washes and planted landscaping such as trees and shrubs.
"It's important to do something with our urban environment to keep us from being concretized to death," said Lenart, an environmental scientist who specializes in climate issues.
Researchers at UA, ASU and elsewhere have known for years that buildings, asphalt streets, concrete sidewalks and other impervious surfaces absorb the sun's heat in the daytime and radiate it outwards at night.
The UA researchers, comparing nighttime temps from a north side, UA experimental farm to those at four rural locations, concluded that the city is warming at night by .070 degrees Fahrenheit per year faster than its rural surroundings.
The urban-rural split is most pronounced from February through June, in part due to the desert's natural cooling effects as new plants emerge in the spring, said the project's lead researcher, Christopher Scott, an associate professor at UA's Udall Center for Studies in Public Policy. At the Tucson International Airport, nighttime temperatures have not risen nearly as fast as at the UA experimental farm at Campbell Avenue near River Road - only by 1 to 1.5 degrees since the 1950s and '60s. That's probably because the airport area and the neighboring Interstate 10 corridor are regularly exposed to a cold air drainage system coming from south of the city, Scott said.
he notices the heat
As a construction worker, Bill Thornton, who is 47, says he can't be totally against the heat island effect because he makes his living from growth.
But he's certainly noticed summertime temperatures are getting hotter.
During the day at his job laying foundations, he wears long-sleeved shirts and sunblock where he never used to a decade ago. On summer nights at home he leaves his air conditioning on for two or three hours, and his summer electric bills have risen nearly 20 percent in recent years.
Barbara Warren, a retired physician and a climate change activist, says that at night with her air conditioner going, even with solar panels on her roof, she has to use more electricity than her panels can produce. She remembers enduring four months of unbearable heat here in the late 1970s; now, it's six months.
The hotter summer temperatures make it harder for her to keep her vegetable garden going, she said.
"I don't see as many bees around my yard in the summer as I used to," Warren said. "I need bees to pollinate, and my plants don't produce fruit. You can grow winter greens here, but in the summer a lot of the things that grow are flowering plants that require pollination to produce."
Bob Vint, an architect who has lived most of his life here, says he notices the heat island's impact when he goes to a neighborhood without paved streets such as Richland Heights near North Campbell Avenue and East Prince Road.
"I had a client with a home in Richland Heights and I went into that part of the city and it was noticeably cooler. There was no asphalt soaking up heat."
Looking at the northeast quarter of a square mile of midtown Tucson, in an area bounded by North Alvernon Way and North Swan Road on the west and east, and by East Speedway and East Broadway on the north and south, Vint calculated that about 2.3 million of 6.9 million total square feet - or 33 percent - is paved. He believes that is a major contributor to the heat island.
"We have unusually wide streets - they're 50 feet wide and have a 75-foot-right of way," said Vint, who lives in that area himself. "We design everything for our convenience, for turning your car around.
"With that and our single-story mentality, you're building a city 40 miles across and one story high. I can't think of a more inefficient city in terms of the energy grid and energy use."
There is a partial antidote to the heat island effect.
The ASU study found that painting roofs white to reflect the sun's heat, dissipating it into the atmosphere, can reduce the heat island impacts of building by 50 percent.
That solution comes with a tradeoff. A second study done this fall by Georgescu and two ASU colleagues found that such "cool" roofs also reduce evapotranspiration of moisture into the air and could knock summertime rainfall down another 4 percent.
Another strategy would be to build roads from more permeable materials, allowing rainfall to be stored there rather than running off into the sewer system, he said. Many scientists, planners, environmentalists and city officials also say people need to plant more, low-water use landscaping to shield their homes and yards from the heat.
Here again, there is a trade-off. Kevin Trenberth, a federal climate scientist in Boulder, Co., says planting and irrigating more trees and other landscaping pumps moisture into the air, aggravating the humidity - a potentially big deal in Tucson, known for its "dry heat."
Trenberth said he's personally experienced that in Boulder, where he works as a senior climate researcher for the National Center of Atmospheric Research. It's still very dry up there, but it used to be that on 90-degree days the temperatures dropped into the low 60s at night, he said.
Now, those same 90-degree days more often drop into the upper 60s at night, which he believes is related to the extra moisture in the atmosphere caused by a growing population using more water on plants. UA researcher Christopher Scott, however, said that on balance, he would opt for the daytime cooling and shade offered by vegetation - making the added humidity "an acceptable tradeoff."
But another common heat adaptation device - the air conditioner - has far less desirable trade-offs, potentially making the heat island effect worse, the two agree. While we run it more and more as the mercury keeps rising, it cools the hot air that comes in and then pushes outside just what we don't need: more warm air.